The term “induction heating” generally describes a process in which an alternating current is passed through a coil to generate an alternating magnetic flux. When the coil is placed in close proximity to or wrapped around a metallic object that is to be heated, the alternating magnetic flux inductively couples the load to the coil and generates eddy currents within the metallic object causing it to become heated. Because of its function, the coil is often referred to as a “work coil” or “induction head,” and the metallic object to be heated as a “load”. Induction heating may be used for many purposes including curing adhesives, hardening of metals, brazing, soldering, welding, and other fabrication processes in which heat is a necessary agent or catalyst.
The field of induction heating is considered to be well-established, with several types of induction heating systems having been developed to control power delivered to the induction head and, thus, the heat produced in the load. One type of induction heating system, sometimes referred to as a resonant system, generally comprises a power supply, a resonant induction head typically formed by the work coil and a capacitor, and some type of switching means to control delivery of power to the resonant induction head by the power supply. Generally, the switching means is closed to cause the power supply to provide a current to the resonant induction head resulting in energy being stored in the work coil. When the switching means is opened, the induction head begins to generate an oscillating voltage and a corresponding oscillating current and alternating magnetic flux, and the stored energy is transferred to the load as heat. If the stored energy is not replenished by the power supply, the oscillating voltage eventually decays to zero, or “rings out,” when all of the stored energy has been transferred to the load.
The greatest amount of energy is transferred from the induction head to the load during a first half-cycle of oscillation of the induction head. Therefore, to achieve maximum heating of a load, induction heating systems replenish the stored energy in the induction head upon completion of the first half-cycle of oscillation. However, maximum heating of a load is not always desirable. When a load requires only low-level heating, some induction heating systems utilize several cycles of the oscillating voltage to heat the load and employ some type of timing mechanism to replenish the stored energy in the resonant induction head after a given time has elapsed. However, the time required for the energy stored in the induction head to dissipate is load-dependent. If the load is smaller than anticipated or has been removed altogether, a substantial amount of stored energy could be remaining in the coil when the energy is replenished resulting in a potentially damaging over-current in the induction head.
Induction heating systems, particularly those employing resonant induction heads, would benefit from a simple low-level heating scheme that protects against potentially harmful over-current of the induction head.